This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.
Nowadays, electronic signals with large bandwidths, above 1 GHz, are relevant to variety of different applications. As part of generating such signals, due to limitations of strictly electronic circuits and lack of such limitations in optical systems, optical-to-electronic conversion is now commonplace. For applications in radar and communications systems, waveforms designed with specific radio frequency characteristics would be useful. In particular, ultra-broadband arbitrary radio frequency (RF) waveforms with large time-bandwidth product (TBWP) are relevant to a variety of these applications including radar imaging and high-data rate covert wireless communications. Due to limits associated with digital-to-analog converters, electronic arbitrary waveform generation (AWGs) have a restricted RF bandwidth. Although recent developments have increased bandwidth approaching 18 GHz, electronic solutions suffer large timing jitter and further it may be difficult to deploy in harsh environments characterized for example by high electromagnetic interference (EMI). On the other hand, photonics approaches are generally immune to EMI, thereby in a position to provide ultra-broad bandwidth and support various applications.
In particular, photonic radio-frequency (RF) arbitrary waveform generation based on spectral shaping and frequency-to-time mapping has received substantial attention. This technique, however, is critically constrained by the far-field condition which imposes strict limits on the complexity of the generated waveforms. While a more in-depth discussion is provided below, in the body of the specification, a brief discussion is provided in this section to assist the reader in understanding the unmet need in this area. Analogy is made to electromagnetic radiation from an antenna for simplicity.
Suppose an electromagnetic radiation emanates from an antenna. The radiation is divided typically into three zones: near field, transition field, and far field. The near field zone is typically divided into two sub-zones: reactive and radiative. For antennas physically smaller than half of the radiated wavelength, the length of the reactive zone is typically identified as wavelength/2π (where wavelength is inversely proportional to frequency of the electromagnetic radiation). The length of the entirety of the near field is one wavelength. The transition zone is typically identified as another wavelength. Therefore, the far field begins at two times the wavelength and extends to infinity. For antennas physically larger than half a wavelength, the near versus far field is defined based on the Fraunhofer distance (Far field distance) which can be defined as 2D2/λ where D is the maximum dimension of the antenna and λ is wavelength. In electromagnetic radiation in the far field, the power (i.e., the intensity) is inversely proportional with square of the distance from the source. More importantly, in the far field zone, the waves are typically uniform and are typically undisturbed by the medium. In major contrast, in the near field, interaction with the medium can cause energy to deflect back to the source and further cause a distortion in the electromagnetic wave that deviates significantly from that found in vacuum. Such distortions are problematic for near field operations.
Similarly in optical systems, near field operations suffer from distortion. In particular, and as will be discussed further in the body of the specification, the bandwidth of an undistorted electrical signal after the photonic signal has been converted to an electrical signal is about 0.25 times the finest optical spectral resolution. This relationship is a consequence of the far field limitation. Therefore, as discussed above, if a high bandwidth electronic signal is desired, the finest spectral resolution can only be 4 times the desired bandwidth. For example, a 40 GHz bandwidth requires the finest resolution to be 160 GHz. However, such a course resolution limits one from exploiting the full time bandwidth product (TBWP) available from modern equipment (e.g. Fourier Transform Pulse Shapers).
Therefore, a new arrangement and method are needed to address the unmet need of generating high fidelity waveforms with radically increased TBWP that do not suffer from the far field limitation.